Neutron storage tube



Aug. 13, 1957 E. E. sHELDoN NEUTRON STORAGE TUBE:

2 Sheets-Sheet l Filed Jun'e l, 1951 INVENTOR:

EDWARD EMANUEL SHELIDON vp E W m W/ 4o n /A .M /U wan B A1g- 13, 1957 E. E. SHELDON 2,802,962

NEUTRON STORAGE TUBE Filed June l, 1951 ZISheetS-Sheet 2 .DUE

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4/ A I INVENToR.

' EDWARD EMANUEL SHELDON States Patent iiice 2,802,962 Patented. Aug. 13, 1957 NEUTRN STORAGE TUBE Edward Emanuel Sheldon, NewI York, N. Y.

Application .inne 1, 1951, Serial No. 229,497

Claims. (Cl. 313-65) This invention relates to an improved method and device for intensifying images and refers more particularly to an improved method and device for intensifying images formed by the impingement of neutrons and other invisible radiations, such as gamma rays and the like, and also irradiation by beams of atom particles, such as e. g. electrons or protons on a uorescent or other reactive screen, and it is a continuation in part of my patent application Serial No. 741,803 for Method and Device for Intensication of Images, tiled April 16, 1947, now U. S. Patent No. 2,555,423. It has also a common subject matter with my patent application, Serial No. 84,327, tiled March 30, 1949, now U. S. Patent No. 2,761,084 and entitled Method and Device for Intensitying Images of Invisible Radiation.

One primary object of the present invention is to provide a methodand device to produce intensified images. This intensification will enable one to overcome the ineiiiciency of the present fluoroscopic examinations. At the present level of illumination of the uoroscopic image, the human eye has to rely exclusively on scotopic (dark adaptation) vision, which is characterized by a tremendous loss of normal visual acuity in reference both to detail and to the contrast.

Another object of this invention is to make it possible to prolong the iuoroscopic examination, since it will reduce markedly the strength of radiation affecting the patients body. Conversely, the exposure time or energy necessary for the radiography may be reduced.

Another object is to provide a method and device to store neutron images, which was not possible until now.

The present intensifying devices concerned with reproduction of neutron images are completely unsatisfactory, because at low levels of uorescent illumination, such as we are dealing with, there are not enough neutrons or protons to be absorbed by fluorescent or photoelectric screens used in such devices. Therefore, the original neutron image can be reproduced by them only with a considerable loss of information. It is well known that the lack of sufficient number of neutrons cannot be remedied by the increase of intensity of neutron radiation, as it will result in damage to the patients body. This basic deciency of the neutron examination was overcome in my invention by using a neutron exposure of a strong intensity but of a short duration, and storing the invisible neutron image for subsequent inspection for the desired length of time without any need of maintaining neutron irradiation. The neutron beam, therefore, can be shut off while reading the stored neutron image and in this way, the total neutron exposure received by the patient is not increased, in spite of using bursts of a great neutron intensity. The storage of neutron images will be also helpful when having weak sources of neutrons. If the examined object is not moving, the neutron exposure, if of suliiciently long duration, will produce satisfactory results in spite of its weakness due to storage.

In order to obtain the objects of this invention, a special neutron sensitive image tube had to be designed, Fig. 1. This novel neutron image tube is characterized by elimination of the optical lens system present in other image tubes, which resulted in 20-30 fold gain in the light reaching the photocathode. Then, by the combined use of a novel photoemissive pick-up system, of a multiplier section of the tube, of a novel electron image amplier system, of the electronic acceleration and of the electronic image diminution and of the storage system, the intensification of the luminosity of the original image exceeding the ratio of 1000-1 was accomplished.`

The elimination of the optical system present in other image tubes to focus the uorescent image on the photocathode of the tube was accomplished by positioning within the neutron sensitive image tube of 'the screen, consisting of combination of neutron transparent, light reflecting layer, of neutron uorescent, or reactive layer, and of the photoemissive layer.l All layers are placed in close apposition to each other to prevent the loss of'denition. The fluorescent and photoemissive layers are separated only by, a very thin light transparent, chemically inactive, barrier layer of a thickness not exceeding 0.15 millimeter. The previous combinations of fluorescent and photoemissive layers were not successful because of detrimental chemical interaction of both layers, due to lack of a barrier between them; therefore, the introduction of light transparent barrier layer represents a very important part of this invention. The photoemissive layer is of semi-transparent type. This layer is characterized by emission of electrons' on the side opposite to the side of the incident light. The photoelectrons emitted from the photoemissive layer in a pattern corresponding to the incident light pattern are' accelerated and focused by means of magnetic and/ or electric fields` on the novel image amplifying system. A

The amplication section of the tube consists of one or a few screens, each of them composed of a very thin lightreflecting, electron pervious layer, of a liuores'cent layer, and of a photoemissive layer in close apposition to each other. It is necessary to include a very thin light transparent, chemically inactive barrier layer between the fluorescent and photosensitive layers, in .order to prevent their chemical interaction, which should be of a thickness not exceeding 0.15 millimeter. The electrons from the pick-up section of the image tube are focused by magnetic or electrostatic fields on the uorescent layer of a screen described above. The luminescence of the liuorescent layer of theamplificationscreen `will cause the emission of electrons from the photoemissive layer of the screen. This process can be repeated a few times, using a few screens described above resulting in 10-100 times intensication of the original electron image.

In another modification to be used in this invention, there is an additional multiplier section, which consists of multipliers and can give an additional intensification of the electron image by secondary electron emission.

The electrons leaving the amplifying section are accelerated by means of high voltage electrostatic fields. The accelerating system can be of a conventional type, well known in the art. Much better results with higher voltages will be achieved with an electrostatic multi-lens system.

Next, the electron image is demagnied, which results in its additional intensification. The electron diminution of the image, in order to gain its intensification is well known in the art; therefore, does not have to be described in detail.

Next, the electron image is stored in the special storage target. The storage of the electron image allows the inspection of the neutron image for a desired time without Y n 3 the needof maintaining neutron irradiation during the reading. This saves so much energy in neutron exposures, that the patients body willi not be impaired even with prolonged examination.

In the reading phase of operation, the stored electron image iis'projec't'ed'on the fluorescent screen at the end of the tube, where it can be viewed by the observer directly, or by rneans of an optical magnifying eye piece, through the light transparent end wall of the tube.A The use fof an optical eye piece vto, magnify optically the electronically diminished image'appearingA on the fluorescent screen, is also well known in the art; therefore, it does not need further description. Y ,'The`combina1ti`on' of the above described features of the neutron sensitive image tube allows obtaining intensification lof th'e original neutron image, which was Vthe primary objective ofjthis invention. Having such a marked intensification of the original neutron image, it will be possible n'ow to use aI much finer grain of fluorescent screens than was practical until now and to improve this way detail and contrast ofthe final image, which was another purpose of this invention.

TheV invention will appear more clearly from the following detailed description when taken in connection with rthe accompanying drawings by way of example only, preferred embodiments of the inventive idea.

' In they drawings:

VVFig. 1 is `a cross-sectional view of the neutron image storage tube'. Y Y Fig. 2fis a cross-sectional View of a modification of the neutron image storage tube;V

Figl 2a is across-sectional view of a modification of thel neutron image storage tube. Fig.` 3 is'Va-.cross-sectional view of the neutron storage image tube in combination with optical system.

Y Fig. 4 is a cross-sectional view of a modification of the neutron image Vstorage tube. I

Fig. 5 is across-sectional view of a modification of the Vneutron. image storage tube.

Fig. 5a is a cross-sectional view of a modification of 'the neutronrimage storage system.

-- Fig; 6` and` 6a are plan views of the storage target.

Fig. 7 is a diagrammatic" View of modification'of the neutronsen'sitive photocathode to be used in neutron Figs; 8 a1`1dt91sliowfaV modification of a neutron sensitive `photocathode.`

The faceila of the imatge'tube lshown inV Fig. 1 must be of a material'V transparent tothe: type of radiation to beV used. l,Inside of the face of the tube, there is a very thin, visible, light-reflecting X-ray transparent layer Z, such as of aluminum, which prevents the loss of light from the adjacent fluorescent layer 3;r 'An extremely thin barrier layer V4,-such.as of vthickness less than 0.15 millimeter, separates the fluorescent screen 3 containing powdered glass, from the adjacent photoemissive layer 5.V The refleeting layer2, the liuorescentglayer 3, the separating layer 4, andphotoemissive layer 5 form together a composite photocathode 5a, which converts neutron images into electron images. `It is obvious Vthat the composite photocathode may be of convex'shape instead of at type. The uorescentlayer 3 and photoemissive layers 5 should be correlated, so that under the iniiuence of neutron radiation used, there is obtained a maximum output of-photoemission. More particularly, the uorescent layer should be' composed of a material having itspgreatest sensitivity to the type of neutron radiation to be used, and the photoemissive material likewise should have its maximum sensitivity to the wave lengthemitted' by the uorescent layer. Fluorescent substances that may be used are'willemite, or -otherzinc silicates, zinc selenides, zinc sulphides, lnaphthalene, anthracene, BaPbSOi, or calcium tungstate, with neutron activators. `Suitable activators for neutron-sensitive phosphors are boron, cadmium, gadolinium, uranium or plutonium. The satisfactoryphotoemissivematerials for the composite photocathode will be caesium oxide,

' caesium oxide activated by silver, caesium with antimony',

storage target'.v

caesium with bismuth or arsenic or antimony, with lithium or potassium. The barrier layer 4 between the fluorescent and photoemissive surfaces can be an exceedingly thin transparent nlm of mica, glass, ZnFz, or organic substance, such as nitrocellulose or gelatine, of silicon, of metal, of a conducting material, such as known -in trade under name Nesa, or of a suitable plastic. The thickness of the separating layer should not exceed 0.15 millimeter. The light transparent separating layer 4 in the photocathode 5a may be deposited on the fluorescent layer 3, so that it doesnt require any support by the walls of the tube. In modification of the composite photocathode, the light transparent separating layer 4 may be attached to the walls of the tube by means of metallic rings and may then provide support for other layers.

It is obvious that photocathode 5a may be flat or convex;vwhen using electrostatic system convex shape is more suitable.

The electron image obtained in the pick-up section 5a is now accelerated by electrode 6 and is transferred to the rst screen 5d of the amplifying section 7, by means of focusing magnetic or electrostatic lields, which are not indicated, since they are well known in the art and would only vserve to complicate the illustration. The amplifying section 7 uses one or a few successively arranged special screens 5d, each of them consisting of yan electron pervious, light-reflecting layer 8,v of a fluorescent layer 9, of light transparent barrier layer 10, and of photoemissive layer 5. Fluorescent substance that may be used are zinc silicates, zinc selenides, zinc sulphide, calcium fluoride, BaPbSOi, or calcium tungstate with or without activators; also organic phosphors such as naphthalene may be used. The satisfactory photoemissive materials will be caesium oxide, caesium oxide activated by silver, caesium with antimony, with bismuth or arsenic or antimony with lithium or potassium. The barrier layer 10 between the liuorescent and photoemissive surfaces can be an exceedingly thin transparent film or mica, glass, ZnFz, or organic substances, such as e. g., nitrocellulose or gelatine, of silicon, of a suitable metal, of a conducting material such as known in trade under name Nesa, or of a suitable plastic. The separating layer 4 should be as thin as possible and should not exceed the thickness of 0.15 millimeter. The amplification achieved by this system results in marked intensification of the original image.

In some applications it may be preferableV toY use in conjunction with amplifying system, the electron multiplier section 11a, consisting of one or a few stages of secondary electron multipliers 12, which serve to intensify further the electronic image. In such a case, the electron image from the pick-up section 5a of the tube is accelerated and focused by means of magnetic or electrostatic fields 6onvthe first stage of the multiplier section. The secondary electrons from the iirst stage are focused the same Way on the second stage of the multiplier section and so on. CsO:Cs or AgzMg multipliers provide a good secondary electron emission.

The electrons emerging from the amplifying section are now accelerated by means of electromagnetic or electrostatic fields 19 to the desired velocity, giving thu-s further `intensification of the electron image. Next, the

electronimage is vdiminishedl by means of electromagnetic or electrostatic lenses 19u tothe desired size, resulting inimage intensification proportional to the square power of the linearl diminutionand is projected on the The use 'of a storage target improves markedly 'signal to noise ratio, 'resulting in lpictures of much better detail and contrast. The storage target Vis shown in Fig. 6 and consists of a "thin perforated sheet of woven conducting Wire screen 41a; On the side of the target opposite to 'the photocathode, there is deposited by evaporation storage material such as BaFz, silica or CaFz,

41b, in such a manner that openings 41e in the target should not be occluded. In some cases `on the side of the target facing thephotocathode, there is deposited by evaporation a thin metal coating.

Between the photocathode and the storage target, in a close 'spacing to the target, there is mounted a tine mesh conducting screen 42. On the side of the storage target, opposite to the photocathode, there is disposed a meshed metal electrode 43, which repels electrons during the writing phase of operation and attracts electrons during the reading phase, Adjacent to said meshed metallic screen, there is disposed, a fluorescent screen 44, provided with a metallic, light-reilecting electron transparent layer 44a, such as of aluminum. The re-I flector electrode 43, during writing, is kept at the potential negative to the photocathode 5a. Therefore, the photoelectrons transmitted through the perforated target, are repelled by said reector electrode and have to fall back on the storage target 41 and deposit thereon varying charges at successive points according to the pattern of neutron image. The best way of operating my system is to have the storage target surface at zero potential or at photocathode potential and then to write on it positive, which means to deposit positive charges. This can be accomplished by adjusting the potential of the surface of the storage target, so that its secondary emission is greater than unity.

The photoelectrons having the pattern of neutron image after passage through openings in the target, are repulsed back by the reflector electrode 43, because in this phase of operation its potential is lower than that of the storage target. The impingement of photoelectron beam causes secondary electron emission from the target 41 greater than unity. The secondary electrons are drawn away by the mesh screen 41a of the storage target, which is connected to the source of positive potential or by an additional collecting electrode. As a result, a positive charge image is formed on the perforated target 4l having the pattern of the original neutron image. This charge image can be stored in the target as long as 50 hours, if the storage material is CaFz.

In the reading phase of operation of my system, the target 41 is scanned by a slow electron beam 50 from the electron gun 52. The electron beam is focused by magnetic or electrostatic fields 49 and is decelerated by the electrode 42, which may be in the form of a ring or of meslied screen. The deecting fields and synchronizing circuit-s are not shown in order not to complicate the drawings. It is obvious that all elds controlling the scanning beam are inoperative during 4the writing phase of the operation. In the same way, the fields controlling the photoelectron beam are not operating during the reading phase of the operation. A part of the scanning electron beam 50a passes through the perforations 41C in the target 41. The charge image on the target controls the passage of the scanning electron beam 50a acting in the similar manner to a grid in the electron tube. The electron beam 5t) in the reading phase of operation passes through the openings in the target 4l, is modulated by the stored charges on it and strikes the fluorescent screen 44, producing thereby a light image having the pattern of the original neutron image. The fluorescent screen 44 is provided with an electron transparent, light-reliecting backing 44a, such as of aluminum. At the same time, the open mesh metallic screen 43 may be used as a collector for electrons to be converted into video vsignals and transmitted to distant receivers. In case no transmission of stored image is desired, the image reproduced on the fluorescent screen 44 can be markedly intensified by using instead of the scanning electron beam, a broad electron beam from the electron gun 52 covering all storage target 41 or a at ribbon scanning beam covering one line of the image. In some cases, when neutron source is very strong, the storage target maybe omitted and the photoelectron beam from the photocathode 5a may! be, after acceleration, electron-optical diminution andamplilication, focused directly on the liuorescent screen 44 `to reproduce a visible image.

Video signals can be obtained not only from the transmitted electrons of the scanning beam, but as well from the electrons 50h of said scanning beam returning to the electron gun. This part of the electron beam is also modulated by the charge image on the target 41, but is of reverse polarity than the transmitted electrons. The non-transmitted electrons of the scanning electron beam return to the multiplier section 46. They are multiplied there, and then are converted into video signals. This arrangement, by using multiplication of electrons, allows a marked intensiiication of video signals. The returning electron beam Stlb contains two groups of electrons; one group is electrons, which are reflected specularly from the target. Another one is electrons, which are reflected non-specularly, which means, scattered. These two groups can be separated from each other before reaching the multiplier. There are many ways to separate these two groups of electrons, all well known in the art. The best method is to introduce an additional helical motion into a primary scanning beam; then the scattered electrons in the returning beam will be on one side of the specularly reflected electrons; therefore, it will be possible to direct scattered electrons into aperture of the multiplier, while stopping the rellected electrons by the edge of the multiplier aperture. The use of the scattered electrons increases markedly sensitivity of the system, because it reduces the inherent shot noise of the scanning electron beam.

Video signals have the pattern of the original neutron image. They are amplified and transmitted by coaxial cable or by high frequency waves to receivers. Receivers may be of various types such as kinescopes, facsimile receivers, in combination with electrographic cameras and others may be used to reproduce images for inspection or recording. The accelerating, focusing and deecting elds, as wall as synchronizing circuits, are not shown, as `they are well known in the art and would only complicate drawings.

After the stored image has been read and no further storage is desired, it may be erased by the use of the scanning `electron beam 5t), by adjusting the potential of the storage target to the value at which the secondary electron emission of its storing surface is below unity. In such a case, the target will charge negatively to the potential of the electron gun cathode. The potential of the reflector in the erasing phase of operation must be more negative than that of the storage target, so that the scanning electron beam will be repelled to the target.

Another form of the invention is illustrated in Fig. 7, wherein a neutron reactive layer 26, preferably from the group boron, lithium, gadolinium and uranium, or of paraiiine is placed within the neutron storage tube to act as the rst layer of the composite photocathode Se. The protons or electrons liberated 4from this layer 26 under the impact of neutron radiation will strike through a thin electron-pervious, chemically inactive, light-reflecting barrier layer 4a, a suitable uorescent layer 27, causing it to liuoresce and activate thereby, a suitable photoemissive layer 29 through the light transport barrier layer 28. In other cases, a neutron reactive layer of cadmium, indium or copper will be more advantageous, becaus-e of its gamma emission, which will cause `fluorescence of the fluorescent layer 27.

In some cases, it may be more desirable, see Fig. 8, to eliminate the lluorescent layer 27 and to cause protons or electrons from the neutron reactive layer 26ato act on adjacent electron emissive layer 29a, such as of beryllium, magnesium or silver, in which case, electron-pervious, chemically inactive barrier layer 30 may be used to prevent chemical interaction of said adjacent layers. vIn other on the storing surface of the storage target lib a photoelectron image.

cases, better` results are achieved by focusing said pro- .tons or electrons' on an electron-ernissive layer 29a with magnetic or electrostatic elds, see Fig. 9. The remaining parts of neutron-sensitive Vstorage tube are the same as shown in Fig. 1. It is obvious that these modifications of neutron-sensitive photocathode may be used as well in neutron storage tubes shown in Figs. l, 2, 4, and 5u.

In another modification of my invention shown in Fig. 2a, the neutron-sensitive pick-up tube 76 has a composite storage target79 shown in Fig. 6a'. The neutron image is converted Yin the photocathode 5b into photoelectron bean'nhavin'g the pattern of the neutron image. The photocathode 5c consists of a neutron reactive layer, such aslo'f gadoliniumfboronor paraffine 90, of a fluorescent layer 3a, such as zinc sulphides, selenides, CaWOi, or BaPbSOi, of a light transparent separating layer 4a, such as of mica, glass, orfNesa, and of a photoemissive layer 'Sgfsuch as of CsOAg or of Cs, K or Li with Bi, As or Sb. The photoelectr'on image is accelerated and focused on the perforated storage target 79. The target consists of a thin perforated light-transparent dielectric, such as glass 83. Also a metallic mesh screen can be used instead of a perforated glass. In such a case, however, a lighttransparent dielectric must b'e deposited on the mesh screen and in such a manner that openings in the screen remain unobstructed. On the side of the glass layer 83 facing the photocathode is deposited a fluorescent layer 81; also in such a manner that the openings in the glass are unobstructed. On said uorescent layer is deposited a light-reflecting layer 80, such as of aluminum. On the side of the dielectric layer 83, which is away from the photocathode, is deposited a photoemissive layer 84, in such a manner that openings in the dielectric layer are not obstructed. The photoelectron beam from the photocathode causes fluorescence of the layer S1. The fluorescent light passes through glass layer 83 and causes emission of electrons from the photo-emissive layer S4. The emitted photoelectrons are led away by adjacent collecting mesh screen 85. As a result, a positive charge image is stored on'the layer 84. This stored charge controls the passage of the scanning electron beam S7 in the same manner as was described above. The transmitted electrons of the scanning beam will strike the fluorescent screen 89 having a light-reecting electron transparent backing 88 and will reproduce therein the neutron image.

'The 'transmitted electrons are focused on the fluorescent screen 89 by means of magnetic or electrostatic fields, which are Well known in the art.

vIt is obvious that instead of composite photocathode 5a, falso a single layer photocathode 5f of a material emitting charged particles when excited by neutrons, such as of gadolinium, lithium, boron or uranium, or parainc, may be used. v This modification is shown in Fig. 2. The operation of neutron image tube 70 is similar to the tube 76 `described above. this tube, the electron image, having the pattern of the neutron image, is deposited as a positive charge image facing the photocathode. In this modification, the reflector electrode 43 is not necessary and may be omitted.

In Vanother modification of my invention shown in Fig. 3, the neutron image is converted into a fluorescent image 32a in the fluorescent screen 31 outside of the neutron image storage tube 33. The fluorescent image is projected by an optical system, preferably of a reflective type-34 on the neutron image tube, having a photocathode 35 of Va material emitting electrons, such as of CsOAg, or of lithium or potassium or antimony or bismuth. The fluorescent image projected on the photocathode produces The rest of the operation of thisV neutron image storage tube is the same as described above, vat explanation of operation of tube 70.

It is obvious that the composite photocathode, the

The only difference being that in the various modifications of their mutual arrangement` come within the scope and spirit of my invention. One of such modifications is shown by Way of example, only in Fig. 4. The neutron image tube 53 operates in the same way as thertube 70, the only difference being the photoelectron image is vprojected on the storage target at an angle which requires the use of arcuate focusing fields. The photoelectric image is stored in the layer 41]: facing photocathode 5a, as a charge image. The stored charge image modulates the passage of electron beam 50, as was explained above. The stored image is reproduced, therefore, in the uorescent screen 44, by the transmittedbeam 50a.

Another modification of neutron image storage tube in which the photocathode and electron gun are disposed f at the opposite ends of the tube is shown in Fig. 5. This arrangement is suitable only for converting the stored neutron images into video signals and cannot be used for immediate reproduction of neutron images in the same tube. In this modification of my invention, shown in Fig. 5, the invisible neutron image of the examined object is converted by the composite photocathode 5a, which has been described above, into a photoelectron image. The photoelectron image is accelerated by the electrode 47 and is focused by the magnetic or electrostatic elds 48 on the perforated storage target 64, also described above. Between the photocathode and the storage target, in a close spacing to the target, there is mounted a fine mesh conducting screen 65. On the side of the storage target, opposite to the photocathode, there is disposed a meshed metal electrode 67. The reflector electrode 67, during writing, is kept at the potential negative to the photocathode 5a. Therefore, the photoelectrons transmitted through the perforated target are repelle-d by said reflector electrode and have tofall back on the storage target 64 and deposit thereon varying charges at successive points according to the pattern of the neutron image. The photoelectron image may be also stored on the side of the storage target facing the photocathode. In such a case, the storing surface should face the photocathode and the mesh electrode 67 is not necessary. The best way of operating my system is to have the storage ltarget surface at zero potential or-at photocathode potential and then to write on it positive, which means to deposit positive charges. This can be accomplished by adjusting the potential ofthe surface of the storage target, so that its secondary emission is greater than unity. The photoelectrons, having the pattern of neutron image after passage through openings in the target, are repulsed back by the reflector electrode 67, because in this phase of operation its potential is lower than that of the storage target. The impingement of photoelcctron beam causes secondary electron emission from the `target 64 greater than unity. The secondary electrons are drawn away by the mesh screen 64a of thc storage target, which is connected to the source of positive potential or by an additional collector electrode. As a result, a positive charge image is formed on the perforated target 64 having the pattern of the original neutron'image. This charge image can be stored in the target as long as 50 hours, if the storage material is CaFz.

In the reading phase of operation of my system, the

e target 64 is scanned by a slow electron beam 62 from the electron gun 72. The electron beam is focused by magnetic or electrostatic fields 71 and is decelerated by the electrode 73, which may be in the form of a ring or of meshed screen. The deecting elds and synchronizing circuit are not shown in order not to complicate the drawing. lt is obvious that all eldscontrolling the scanning beam are not operating during the reading phase of the operation. A part of the scanning electron beam passes through the perforations in the target 64. VThe charge image on the target controls the passage of the scanning electron beam 62 acting in the similar manner to a grid in the electron tube. The scanning electron beam 62 is, therefore, modulated by the stored charge image. A part of it, 62a, is transmitted through the openings in the storage target 64, is collected by the mesh electrode 65 and is converted into video signals in the usual manner. Another part 62b of the scanning electron beam is returning to the electron gun 72, is diverted to the multiplier 63 and after multiplication therein is converted into video signals. Video signals have the pattern of the original neutron image and are amplified and transmitted by coaxial cable or by high frequency waves to receivers. Receivers may be of various types such as kinescopes, facsimile receivers, in combination with electrographic cameras, and others may be used to reproduce images for inspection or recording.

In some cases, it may be more desirable to have the uorescent screen 44 mounted outside of the vacuum tube; in such cases, thin electron transparent layer of chromium or aluminum is placed on the end wall 22 of the vacuum tube made of fernico glass. The image appearing on the fluorescent screen can be viewed directly or by means of an optical eyepiece giving the desired optical magnification of the image. In other cases, the fluorescent screen 44 is substituted by photographic layer or by photographic layer in combination with uorescent screen, or by an electrographic plate, permitting thus to obtain a permanent record of electron image. The storage of neutron images may be also accomplished by using the reproduced uorescent image on the screen 44 of neutron image tube 95, shown in Fig. a. The uorescent image, having the pattern of neutron image, is projected by means of an optical system 92 on the storage tube 93, having a light sensitive 94 photocathode, such as of CsOAg, Cs, Li or Rb, with As, Bi or Sb, and is converted therein into an electron image. The electron image is focused on the perforated storage target 41 and is stored therein. The stored image is released by scanning it with an electron beam 50 and is reproduced on the uorescent screen 44 in the storage tube 93 for inspection or recording, as was explained above and illustrated in Figs. 1-5.

In another alternative of this invention, the neutron image tube is curved and the electron beam is deected by proper magnetic or electrostatic fields. This arrangement will prevent the positive ions from reaching the photoemissive section.

It will thus be seen that there is provided a device in which the several objects of this invention are achieved and which is well adapted to meet the conditions of practical use.

AJ various possible embodiments might be made of the above invention and as various changes might be made in the embodiments above set forth, it is to be understood that all matter herein set forth, as shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

What I claim is:

1. A vacuum tube comprising in combination a photo cathode for receiving an image, an apertured light-stable screen, means for producing a scanning electron beam to scan with said beam said apertured screen and a uorescent screen for receiving electrons of said scanning beam transmitted through said apertured screen, said liuorescent screen comprising furthermore a light reflecting layer on the side facing said electron beam.

2. A device as described in claim l in which said photocathode is light-sensitive.

3. A device as described in claim 1 which comprises in addition means for decelerating said electron beam.

4. A device as dened in claim 1 which comprises in addition means for converting electrons of said scanning beam into electrical signals.

5. A device as defined in claim 1, in which said vacnum tube comprises in addition a fluorescent layer for receiving an image and producing luorescentlight when impinged by said image.

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